Installation for etching objects

ABSTRACT

An installation for etching objects comprises at least one etching machine, in which metal is etched from the objects treated, the etching medium being enriched with metal. The etching medium is regenerated in at least one electrolytic cell by removing metal. Two buffer tanks are located in the lines which connect the etching machine to the electrolytic cell. A first control circuit ensures a substantially constant density of the etching medium in the etching machine. This takes place by the supply of depleted etching medium from the first buffer tank to the etching machine and by the removal of enriched etching medium from the etching machine into the second buffer tank. A second control circuit ensures a substantially constant density in the electrolytic cell by the supply of enriched etching medium from the second buffer tank into the electrolytic cell and by the removal of a corresponding quantity of depleted etching medium from the electrolytic cell into the first buffer tank. Both control circuits operate independently of each other.

The invention relates to an installation for etching objects, inparticular printed circuit boards, with

a) at least one etching machine, in which metal is etched from theobjects, the etching medium being enriched with metal;

b) at least one electrolytic cell, in which enriched etching medium isdepleted;

c) at least one electronic control circuit, which controls the exchangeof etching medium between the etching machine and the electrolytic cellso that the density of the etching medium in the etching machine issubstantially constant.

In known etching installations of this type, only a single controlcircuit is provided, which attempts to keep the density of the etchingmedium in the etching machine constant by appropriate operation of theelectrolytic cell. This control system operates relatively slowly, sothat this may lead to considerable shortages or excesses of the metalconcentration in the etching medium located in the etching machine.Moreover this results in a discontinuous operation with frequentstarting and stopping of the electrolytic cell, which has anunfavourable influence on the quality of the copper separated. Looseningof the copper layer may also occur, which may lead to short-circuits.

It is the object of the present invention to develop an etchinginstallation of the aforementioned type so that the density of theetching medium in the etching machine as well as of the electrolyticcell can be kept at a constant value with low hysteresis and highaccuracy.

An etching installation according to the invention, by which this objectis achieved, is characterised by

d) a first buffer tank, which is located in the connecting line betweenthe outlet of the electrolytic cell and the inlet of the etchingmachine;

e) a second buffer tank, which is located in the connecting line betweenthe outlet of the etching machine and the inlet of the electrolyticcell;

f) a first control circuit, which monitors the density of the etchingmedium in the etching machine and on exceeding a predetermined densityvalue supplies depleted etching medium from the first buffer tank to theetching machine and removes a corresponding quantity of enriched etchingmedium from the etching machine into the second buffer tank;

g) a second control circuit, which monitors the density of the etchingmedium in the electrolytic cell and on falling below a predetermineddensity value supplies enriched etching medium from the second buffertank to the electrolytic cell and removes a corresponding quantity ofdepleted etching medium from the electrolytic cell into the first buffertank,

so that the two control circuits are isolated from each other by thebuffer tanks and operate independently of each other.

Thus, according to the invention the electrolytic cell operatescompletely independently of the etching machine. On account of thebuffer tanks provided, the quantities of depleted etching mediumrequired for maintaining the correct density can be supplied to theetching machine in a problem-free manner and at any time, independentlyof whether the electrolytic cell makes or does not make available justdepleted etching medium. In a corresponding manner enriched etchingmedium can be removed from the etching machine independently of whetherthe electrolytic cell is ready to receive enriched etching medium ornot. In turn, the electrolytic cell may operate completely independentlyof the yield of etching medium to be depleted from the etching machineor of the requirement of the etching machine for depleted etchingmedium. It is stopped when the second buffer tank is empty. Starting andstopping of the electrolytic cell is required solely at relatively greatintervals due to the buffer action of the buffer tanks.

For safety reasons, the electrolytic cell should also be shut down inthe case of a second density value, which lies below the above-mentioneddensity value.

It is appropriate if the electrolytic cell is put out of operationwhilst voltage is applied, by switching off the pump, which circulatesthe etching medium through the electrolytic cell, the contents of thecell flowing into the associated storage tank.

If, as explained in claim 4, the etching medium supplied to the etchingmachine from the first buffer tank and the etching medium supplied tothe second buffer tank from the etching machine are passed through aheat exchanger, energy is saved. The etching medium introduced from thefirst buffer tank into the etching machine must be brought to theworking temperature at that point; the necessary heat can be removedfrom the etching medium to a considerable extent, which is removed fromthe etching machine into the second buffer tank.

In this case it is once more an advantage if the opening point of thepipe, by which etching medium is removed from the etching machine, islocated at the height of the operating level of the sump of the etchingmachine, and if the delivery capacity of the pump, which removes etchingmedium from the etching machine, is slightly above the delivery capacityof the pump, which supplies etching medium to the etching machine. Inthis way it is automatically ensured, without special additionalmeasures such as level sensors or the like, that the filling height inthe pump sump always remains constant. Due to the higher deliverycapacity of the discharging pump, the filling height can never riseabove the opening point of the removal pipe.

In etching installations of the aforementioned type, but to a greaterextent in the design according to the invention, liquid losses occur dueto evaporation. The latter must be compensated for. For this purpose,according to one feature of the invention, a water-control unit isprovided, which keeps the sum of the filling heights in the varioussumps, containers and tanks constant by the addition of fresh water.

As regards switching technology and apparatus, that embodiment isparticularly favourable, in which

a) the filling heights in the buffer tanks are monitored by levelsensors, which are connected to the water control unit;

b) the sum of the filling heights in the buffer tanks is kept constantby the addition of fresh water;

c) the filling heights in the other parts of the installation are keptconstant independently of the addition of fresh water.

Generally the above condition c) is fulfilled anyhow by the method ofconstruction of the etching cell (see also the embodiment of theinvention according to claim 5). In this case, thus solely the fillingheights in the two buffer tanks need to be monitored, to which thecorresponding quantity of fresh water is then supplied.

Now in order to avoid that the supply of fresh water to the buffer tanksleads to undesirable dilutions of the etching medium contained therein,the addition of fresh water to each of the buffer tanks advantageouslytakes place in proportion to the filling heights of these buffer tanks.The fuller buffer tank thus receives a larger quantity of fresh waterthan the emptier buffer tank, so that the dilution by fresh water inboth buffer tanks is approximately the same.

One particular advantage of the use of buffer tanks according to theinvention consists in that according to claim 8 the capacity of theelectrolytic cell can be less than the capacity of the etching machine.In this case, allowances are made for the fact that generally the dailyoperating time of the etching machine corresponds to the normal workingtime during operation (for example 8 hours), whereas the electrolyticcell can be operated without problems around the clock, thus 24 hours aday. The capacity of the buffer tanks must be dimensioned to correspondto the different operating times of the electrolytic cell and of theetching machine.

One embodiment of the invention will be described in detail hereafterwith reference to the drawings; the single FIGURE shows diagrammaticallyan installation for etching objects.

The installation for etching objects illustrated in the drawingcomprises as its main components an etching machine 1, a dosing unit 2,a first buffer tank 3, a second buffer tank 4, an electrolytic cell 5 aswell as a storage tank 6 for etching medium discharged from theelectrolytic cell 5.

The construction of the etching machine 1 is basically known: theobjects 7 to be etched are moved by the continuous method from an inlet8 to an outlet 9 of the etching machine on a roller conveying system 10.They thus pass an upper nozzle assembly 11 as well as a lower nozzleassembly 12, by which they are sprayed with etching medium. The latteris supplied to the nozzle assemblies 11, 12 by a pump 13, which isconnected on the suction side to the sump 14 of the etching machine. Theetching medium drips from the objects 7 to be etched back into the sump,in which case it changes its chemical composition on account of theetching process and due to evaporation processes.

The dosing unit 2 is provided for monitoring and controlling thechemical composition of the etching medium in the etching machine 1. Thesump 14 of the etching machine 1 is connected by way of a connectingline 15 to a container 16 of the dosing unit 2. A pump 17 removesetching medium continuously from the container 16 and conveys the latterin the circuit by way of a ph-meter 18 and a density-measuring device 19back into the container 16. The delivery side of the pump 17 is alsoconnected to two injectors 20, 21, in which NH₃ is mixed with theflowing etching medium. In the left-hand injector 20 in the drawing, NH₃is added to the flowing etching medium, which NH₃ originates from astorage tank and its flow is determined by a solenoid valve 22. Thesolenoid valve 22 is in this case controlled electrically by theph-meter 18. A minimum ph-value of the etching medium in the etchingmachine 1 is thus ensured by means of the ph-meter 18 due to theaddition of NH₃ by way of the solenoid valve 22.

By way of the right-hand injector 21 in the drawing, the gas sucked fromthe electrolytic cell 5, which contains substantially ammonia, isreturned to the etching medium. In this way, the evaporation losses ofNH₃ are kept small and environmental problems are reduced.

By means of the above-mentioned density-measuring device 19, the densityof the etching medium in the etching machine 1, which would rise withoutspecial precautions due to the metal etched from the objects 7 (in thecase of printed circuit boards generally copper), is kept at a constantvalue. This takes place in the following manner:

The left-hand buffer tank 3 in the drawing contains a supply of etchingmedium, which was supplied from the electrolytic cell 5 in a manner tobe described further hereafter. On the other hand, the right-hand buffertank 4 in the drawing contains etching medium of higher density enrichedwith copper, which is supplied to the electrolytic cell 5 for depletionin a manner likewise to be described hereafter.

A pump 23 is connected on the suction side by way of a line 24 to thebuffer tank 3. It conveys the depleted etching medium removed from thebuffer tank 3 by way of a heat exchanger 25 into the sump 14 of theetching machine 1. A further pump 26 is connected by way of a line 27 onthe suction side to the sump 14 of the etching machine 1. The openingpoint of the line 27 is located at a height which corresponds to theoperating level of the sump 14 in the etching machine 1. The pump 26passes the etching medium removed from the sump 14 of the etchingmachine 1 likewise through the heat exchanger 25, where a heat exchangetakes place between the etching medium supplied to the sump 14 and theetching medium removed from the sump 14. The etching medium conveyed bythe pump 26 then flows from the heat exchanger 25 into the second buffertank 4, in which, as mentioned above, etching medium enriched withcopper is located.

The pumps 23 and 26 are connected to each other electrically or--asillustrated--mechanically by a common motor. The arrangement is suchthat both pumps 23, 26 are always operated simultaneously, the deliverycapacity of the pump 26 always being kept somewhat higher than thedelivery capacity of the pump 23. In this way it is ensured that theoperating level of the etching medium in the sump 14 of the etchingmachine 1 is always determined by the opening point of the line 27 intothe sump 14.

The right-hand buffer tank 4 in the drawing is connected by way of aline 28 to the suction side of a pump 29, which is connected on thepressure side by way of a non-return valve 30 and a flow meter 31 to thesump 32 of the electrolytic cell 5.

The overflow 33 of the electrolytic cell, from which the depletedetching medium flows, is connected by way of a line 34 to the left-handfirst buffer tank 3 in the drawing. A further line 36, in which asolenoid valve 37 is connected, leads from the overflow 34 of theelectrolytic cell 5 into the storage tank 6. The sump 38 of the storagetank 6 is connected by way of a line 39 to a pump 40, which supplies theetching medium removed from the sump 38 by way of a flow meter 41, withwhich a non-return valve 42 is connected in parallel, to the sump 32 ofthe electrolytic cell 5. The pump 40 is also connected on the deliveryside to a solenoid valve 43, which controls the flow path to adensity-measuring device 44. The etching medium flowing through thedensity-measuring device 44 is guided back to the storage tank 6.

Located parallel to the density-measuring device 44 is a hydroxidefilter 45, the flow through which can be released as required by meansof a valve 46.

The electrolytic cell 5 and storage tank 6 are operated and controlledin the following manner:

At the beginning of operation, the electrolytic cell 5 must be filledwith etching medium from the storage tank 6. This takes place by meansof the pump 40. When the electrolytic cell 5 has reached its fillinglevel, this is ascertained by a level sensor 47, which opens thesolenoid valve 43. Due to this a flow by-pass is released, which reducesthe flow of etching medium from the storage tank 6 into the electrolyticcell 5 to the amount necessary for continuous operation. A major part ofthe etching medium conveyed by the pump 40 now flows through thedensity-measuring device 44 and through the hydroxide filter 45 back tothe storage tank 6.

Normally, the solenoid valve 37 is open. This means that etching mediumis circulated by the pump 40 by way of the electrolytic cell 5, itsoverflow 34 and the solenoid valve 37. However, if etching medium passesfrom the buffer tank 4 into the sump 32 of the electrolytic cell, thelevel in the sump 38 of the storage tank rises. A level switch 48records the rise of the liquid level in the sump 38 and closes thesolenoid valve 37. Depleted etching medium now flows through the line 35into the buffer tank 3.

The density-measuring device 44 monitors the copper content of theetching medium circulated by the pump 40. If this copper content dropsbelow a predetermined value, for example below 30 g/l, the pump 29 isset in operation. On account of the above-described operations, in thiscase a corresponding quantity of etching medium is removed from theelectrolytic cell 5 and supplied to the buffer tank 3. The supply ofenriched etching medium from the buffer tank 4 increases the density ofthe etching medium in the electrolytic cell 5 until thedensity-measuring device 44 once more stops the pump 29.

The electrolytic cell 5 is put out of operation when the buffer tank 4is empty. This takes place by switching off the pump 40. The contents ofthe electrolytic cell thus flow by way of the flow meter 41 and mainlyvia the non-return valve 42, line 39 and pump 40 back into the storagetank 6. However, the electrolytic cell 5 remains supplied with voltage.

For safety reasons, the electrolytic cell 5 is always shut down when thedensity of the etching medium contained therein falls below a secondvalue, which lies below the above-mentioned control point.

The entire installation described above obviously contains two controlsystems, which are isolated from each other by the two buffer tanks 3,4:

The density-measuring device 19 operating in the first control systemensures a constant density of the etching medium in the etchingmachine 1. The constant density is brought about by the supply ofdepleted etching medium from the buffer tank 3 or by the removal ofenriched etching medium to the buffer tank 4. On account of theexistence of the buffer tanks 3, 4, independent of the respectivefunction of the electrolytic cell 5, depleted etching medium or spacefor enriched etching medium is always available. The first controlsystem, which contains the density-measuring device 19 as its "core",can also operate completely "autonomously".

The second control system contains the density-measuring device 44 asthe controlling unit. It ensures that the density and thus the coppercontent of the etching medium in the electrolytic cell 5 is kept at apredetermined value. This takes place, as described above, by switchingthe pump 29 on and off. Once more this control system is completelyisolated from the first control system, which contains the etchingmachine 1, since the electrolytic cell 5 may discharge depleted etchingmedium into the buffer tank 3 independently of the instantaneousrequirement. Likewise, enriched etching medium can be supplied from thebuffer tank 4 to the electrolytic cell 5 according to the requirementsof the control circuit governing this, irrespective of whether accordingto the state in the etching machine 1, etching medium just enrichedthere is available or not.

Due to the aforedescribed isolation of the two control systems, it ispossible to regulate and keep constant the density of the etching mediumat the actually critical point, namely in the etching machine 1 itself,with greater precision than could be achieved when using solely onecontrol system, which covers both the etching machine 1 as well as theelectrolytic cell 5. However, the two buffer tanks 3 and 4 can be usedin a further advantageous manner, which will now be described in detail:

In known etching installations, the operating times of the etchingmachine and of the electrolytic cell 5 used for regeneration of theetching medium are identical daily. If one uses buffer tanks 3 and 4, asdescribed above, then the operating times can be kept different. In thisway, a smaller capacity of the electrolytic cell 5 is adequate; it nolonger needs to be adapted to the peak requirement of the etchingmachine 1.

One example:

The etching machine 1 is designed so that it etches away 9 kg Cu perhour, i.e. 72 kg in an 8-hour working day. In order to recover the samequantity of copper in an electrolytic cell 5, which works for 24 hours aday, a capacity of 3 kg Cu/hour is sufficient for the latter. If thedepletion in the electrolytic cell amounts to 50 g Cu/1, this means that1440 l etching medium must be buffered. However, since in the 8 hours ofthe operating time of the etching machine 1, approximately 480 l arerecovered from the electrolytic cell 5, each buffer tank 3, 4 must havea capacity of approximately 1000 l.

The continuous method of operation of the electrolytic cell 5 possiblewhen using the buffer tanks 3, 4 not only reduces the expenditure forapparatus for the entire etching installation; in addition it improvesthe regeneration operation in the electrolytic cell 5.

During the operation of each etching installation, but particularly whenusing buffer tanks 3, 4, liquid losses due to evaporation occur. Theseliquid losses must be compensated for. In the above-described etchinginstallation, this takes place as follows:

The levels in the buffer tanks 3, 4 are monitored continuously by levelsensors 49 or 50. The latter are connected to an electronicwater-control unit 51. The latter in turn controls a first solenoidvalve 52, by way of electrical leads, which are shown in broken line inthe drawing, which solenoid valve controls the supply of fresh water tothe left-hand buffer tank 3 in the drawing, as well as a second solenoidvalve 53, which controls the supply of fresh water to the right-handbuffer tank 4 in the drawing.

Due to the above-described method of operation of the etching machine 1and of the electrolytic cell 5, it is ensured that a constant fillingheight is guaranteed therein, without special measures. It is thereforesolely necessary for this to ensure that the sum of the levels in thetwo buffer tanks 3, 4 likewise remains constant. This is the very taskof the water-control unit 51. If the latter ascertains a drop of thelevels in the buffer tanks 3, 4 in such a way that the sum thereof fallsbelow a reference value, it opens the solenoid valves 52, 53 until thesum of the levels has again reached the desired value. In order toobviate undesired dilutions of the etching medium, which could disturbthe operation of the various control circuits, the addition of water toeach of the buffer tanks 3, 4 takes place in proportion to therespective level in these tanks. Thus, if for example the level in theleft-hand buffer tank 3 in the drawing is twice as high as in theright-hand buffer tank 4 in the drawing, the addition of water by thewater-control unit 51 takes place in such a way that twice as much wateris supplied to the left-hand buffer tank 3 as to the right-hand buffertank 4.

I claim:
 1. Installation for etching objects, in particular printedcircuit boards, witha) at least one etching machine, in which metal isetched from the objects, the etching medium being enriched with metal;b) at least one electrolytic cell, in which enriched etching medium isdepleted; c) at least one electronic control circuit, which controls theexchange of etching medium between the etching machine and theelectrolytic cell so that the density of the etching medium in theetching machine is substantially constant;characterised by d) a firstbuffer tank (3), which is located in the connecting line (35) betweenthe outlet of the electrolytic cell (5) and the inlet of the etchingmachine (1); e) a second buffer tank (4), which is located in theconnecting line (28) between the outlet of the etching machine (1) andthe inlet of the electrolytic cell (5); f) a first control circuit (15,16, 17, 19, 23, 26), which monitors the density of the etching medium inthe etching machine (1) and on exceeding a predetermined density valuesupplies depleted etching medium from the first buffer tank (3) to theetching machine (1) and removes a corresponding quantity of enrichedetching medium from the etching machine (1) into the second buffer tank(4); g) a second control circuit (40, 43, 44), which monitors thedensity of the etching medium in the electrolytic cell (5) and onfalling below a predetermined density value supplies enriched etchingmedium from the second buffer tank (4) to the electrolytic cell (5) andremoves a corresponding quantity of depleted etching medium from theelectrolytic cell (5) into the first buffer tank (3),so that the twocontrol circuits are isolated from each other by the buffer tanks (3, 4)and operate independently of each other.
 2. Etching installationaccording to claim 1, characterised in that on falling below a seconddensity value, which is below the first density value, the electrolyticcell (5) is put out of operation.
 3. Etching installation according toclaim 1 characterised in that by switching off the pump (40), whichcirculates the etching medium through the electrolytic cell (5), theelectrolytic cell (5) is put out of operation while voltage is appliedto the electrodes.
 4. Etching installation according to claim 1,characterised in that the etching medium supplied to the etching machine(1) from the first buffer tank (3) and the etching medium supplied tothe second buffer tank (4) from the etching machine (1) are passedthrough a heat exchanger (25).
 5. Etching installation according toclaim 1, characterised in that the opening point of the line (27), bywhich the etching medium is removed from the etching machine (1), islocated at the height of the operating level of the sump (14) of theetching machine (1) and that the delivery capacity of the pump (26),which removes etching medium from the etching machine (1), is slightlygreater than the delivery capacity of the pump (23), which suppliesetching medium to the etching machine (1).
 6. Etching installationaccording to claim 1, characterised in that a water control unit (51) isprovided, which keeps the sums of the filling heights in the varioussumps (14, 32), containers (16, 38) and tanks (3, 4) of the etchinginstallation constant by the addition of fresh water.
 7. Etchinginstallation according to claim 6, characterised in thata) the fillingheights in the buffer tanks (3, 4) are monitored by level sensors (49,50), which are connected to the water control unit (51); b) the sum ofthe filling heights in the buffer tanks (3, 4) is kept constant by theaddition of fresh water; c) the filling heights in the other parts ofthe installation are kept constant independently of the addition offresh water.
 8. Etching installation according to claim 7, characterisedin that the addition of fresh water to each of the buffer tanks (3, 4)takes place in proportion to the filling heights of these buffer tanks(3, 4).
 9. Etching installation according to claim 1, characterised inthat the capacity of the electrolytic cell (5) is less than the capacityof the etching machine (1).